In the production process of a photomask having a pattern of an electronic circuit which is formed from a light blocking material on a semiconductor device such as a semiconductor integrated circuit (LSI) or a transparent substrate, or an imprint mold which is a heat or light imprint mold part, microfabrication is carried out by lithography using a photoresist. This is carried out by forming a photoresist thin film on a silicon substrate or a quartz glass substrate having a light blocking thin film thereon, selectively applying a high-energy beam such as an excimer laser beam, X-ray or electron beam to only part of the photoresist thin film so as to form a pattern latent image, developing it to obtain a resist pattern and etching the substrate by using the pattern as a mask.
Stated more specifically, in photolithography, an organic solvent solution of a photosensitive polymer material called “resist composition” is applied to a substrate having a layer to be processed on the surface and prebaked to evaporate the organic solvent so as to form a resist film. Then, the resist film is partially irradiated with light, and further unwanted parts of the resist film are dissolved and removed by using a developer to form a resist pattern on the substrate. Thereafter, the layer to be processed on the substrate having this resist pattern as a mask is dry etched or wet etched. Finally, unwanted parts are removed from the resist film to complete microfabrication.
In the production process of a photomask and an imprint mold, a pattern is formed by using an electron beam drawing apparatus or a laser drawing apparatus in most cases. As for a semiconductor device such as LSI to be formed on the silicon substrate, studies into the formation of a pattern using an electron beam drawing apparatus have been started for the further miniaturization of devices. Therefore, the development of a process using an electron beam resist is now actively under way. This electron beam resist is desired to have high etching resistance, high resolution and high sensitivity. Further, as pattern collapse readily occurs at the time of drying after development or rinsing along with the miniaturization of patterns, a process free from the occurrence of pattern collapse is desired.
Various types of organic resists which are sensitive to electron beams are known, and a resist pattern is formed by various methods. For example, there is proposed a method of forming a fine pattern by forming a thin film of a polymer of an ethylenically unsaturated monomer such as polymethyl methacrylate on a substrate as a resist film, irradiating it with an electron beam to form a predetermined image and developing the image with a low-molecular weight ketone such as acetone (refer to JP-A 8-262738). There is also proposed a method of forming a fine pattern by forming a thin film of a resist material containing a calixarene derivative as a resist film, irradiating it with an electron beam to form a predetermined image and developing the image with ethyl lactate, propylene glycol monomethyl ether or 2-heptanone (refer to WO2004/022513). There is further proposed a method of forming a fine pattern by forming a thin film of an electron beam resist material as a resist film, irradiating the film with an electron beam to form a predetermined image and developing the image with a supercritical fluid (refer to Japanese Patent No. 3927575).
Although a fine pattern can be formed according to the method of JP-A 8-262738, since the polymer of an ethylenically unsaturated monomer such as polymethyl methacrylate has low etching resistance, when the layer to be processed is etched deeply by using this resist as a mask, the aspect ratio of the resist pattern must be made large to increase the pattern height. Further, since the developer is a low-molecular weight ketone, the ignition point is low and an explosion-proof apparatus must be installed.
Meanwhile, since the resist material containing a calixarene derivative is used according to the method of WO2004/022513, though a pattern having high etching resistance and a width of 10 nm or less can be formed, this resist material has lower sensitivity than other resist materials and the amount of exposure must be made large. Therefore, there is room for improvement.
For example, FIG. 2 of the pamphlet of WO2004/022513 shows exposure characteristics (sensitivity curve) when the resist material is exposed to a 50 kV electron beam and developed with ethyl lactate or xylene. According to the exposure characteristics, the sensitivity of the resist used in the method of the above pamphlet is about 1 to 2 (mC/cm2) and the sensitivity of a calix[4] arene-based resist having especially high resolution is about 2 (mC/cm2). Higher sensitivity is required for the practical use of the resist. According to the pamphlet, the sensitivity of the resist is expressed by the minimum amount of exposure (mCcm−2) which ensures that the film thickness of a resist pattern obtained after development becomes equal to the standard film thickness which is the film thickness of a resist before development (the thickness of a resist film after the resist is applied and prebaked as required).
According to the method of Japanese Patent No. 3927575, since the supercritical fluid having low surface tension is used at the time of development, it is possible to prevent pattern collapse at the time of drying after development or rinsing. However, an expensive apparatus is required and the throughput is low. Therefore, there is still room for improvement.